Summary:
Now we collect compaction stats per column family, but report default colum family's stat as compaction stats for DB.
Fix it by reporting compaction stats per column family instead.

Test Plan: Run db_bench with --num_column_families=4 and see the number fixed.

Reviewers: IslamAbdelRahman, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D57063